Chemoresistor type gas sensor having a multi-storey architecture

ABSTRACT

A multi-storey gas sensor is constructed by stacking chemoresistor type gas sensing elements and providing holes through each sensing element so gas can pass from one sensing element to the next, through the sensing layers. A rich data set can be obtained by selecting appropriate combinations of materials for the different sensing layers and varying the operating conditions of the different gas-sensing elements by: taking measurements when different combinations of sensing layers are activated, when given sensing layers are heated to different temperatures or according to different heating profiles, and/or when selected sensing layers are exposed to UV light. Sensor sensitivity and selectivity can be increased by applying UV pulses of controlled duration, and target gas species can be detected based on the transient response of the sensing layer at onset of UV irradiation. Each sensing element may have a micro-hotplate architecture.

FIELD OF THE INVENTION

The present invention relates to the field of gas sensors and, moreparticularly, to gas sensors of chemoresistor type.

BACKGROUND OF THE INVENTION

Gas sensors are used in many applications, notably in situations whereit is desired to detect or recognise a particular gas and in situationswhere it is desired to determine the composition of a gas mixture. Inthe present text, unless the context demands otherwise: the expression“gas” will be used to designate both a specific gas species and amixture of different gaseous species, and the general expression“characterisation” will be used to designate both the process ofrecognizing or detecting a particular gas and the process of determiningthe composition of a gas. It is to be understood that references in thistext to a “gas sample” generally include references to any gas which ispresented to the gas sensor (whether as a discrete sample or by exposingthe sensor to an ambient gaseous medium).

Gas sensors have been developed using different sensing technologies,including chemoresistor type gas sensors, such as those based onsemi-conducting metal-oxides. FIG. 1 is a cross-sectional view whichillustrates, schematically, the basic structure of a semi-conductingmetal-oxide type gas sensor 1.

As shown in FIG. 1, a semi-conducting metal-oxide type gas sensor 1 hasa sensing layer 2 made of semi-conducting metal-oxide provided on aninsulating layer 3 supported on a base 4. When the sensor 1 is exposedto a gas, gas particles G may become adsorbed on the surface of thesensing layer 2, and oxidation-reduction reactions may occur, leading toa change in the impedance (conductance, capacitance, inductance orplural of these parameters) of the sensing layer 2. This change ofimpedance is measured using a pair of measuring electrodes 5 provided incontact with the sensing layer 2. Often the measurement is made byapplying a potential difference across the measurement electrodes andmonitoring how the impedance presented by the sensing layer changes. Thewaveform of the signal produced by the measuring electrodes ischaracteristic of the gas reacting with the sensing layer 2 and,typically, the waveforms produced by gases of interest are learnedduring a teaching phase preparatory to analysis of unknown gas samples.

In general, it is necessary to heat the sensing layer 2 to a relativelyhigh temperature (notably 250° C. or above depending on the materialforming the sensing layer and the gas species to be detected) for usefuladsorption phenomena to be observed. Accordingly, typical gas sensors ofthis type also include a heater 6, as well as a temperature sensor (notshown in FIG. 1). After a measurement has been taken the heater 6 isactivated to heat the active layer to a high temperature, above theusual operating temperature, so as to cause de-sorption of adsorbedparticles, thus cleaning the sensor 1 ready for a subsequentmeasurement.

An aim in this field is to be able to construct micro-sensors, that isto say, miniaturized gas sensors particularly those that are smallenough to be integrated into everyday appliances (for example, mobiletelephones, face masks, intelligent toys, etc). It is a requirement formicro-sensors that they should have sufficiently high performance, thatis, they should be able to detect a target gas, and/or determine acomposition of a gas mixture, rapidly and with a sufficiently highdegree of accuracy.

Semi-conducting metal-oxide gas sensors attract particular interest forimplementation as micro-sensors because they can be built inminiaturized form using techniques known from the field of integratedcircuit manufacture.

In recent years semi-conducting metal-oxide type gas sensors having a“micro-hotplate” structure have been developed. FIG. 2( a) is across-sectional view which illustrates, schematically, the generalstructure of a semi-conducting metal-oxide type gas sensor 10 having amicro-hotplate structure. It will be seen from FIG. 2( a) that the base14 of the sensor 10 has a hollowed-out portion 17 so that the sensinglayer 12 is no longer positioned in registration with a thick portion ofthe base 14. Accordingly, the heater 16 which is used to heat thesensing layer 12 only needs to heat a reduced mass of material(including a relatively thin supporting membrane M), which reduces thepower consumed by the gas sensor as well as enabling the temperature ofthe sensing layer 2 to be increased rapidly (thus reducing the timenecessary for making a measurement and reducing the time necessary forcleaning the sensing layer). Moreover, this rapid heating causes lessdamage to the material forming the sensing layer.

FIGS. 2( b) and 2(c) illustrate sensors having two different types ofmicro-hotplate architectures.

In the sensor 20 of FIG. 2( b), the sensing layer 22 is formed on aninsulating layer 23 which, in its turn, overlies the base 24. Conductors28 lead out from the measuring electrodes and heater to make contactwith electrode pads 29 provided on the base 24. Additional wiring (notshown) connects the electrode pads to further circuitry, notably asource of current for the heater, and circuitry for processing thesignals measured by the measurement electrodes. The sensor 20 of FIG. 2(b) has a “closed” type of architecture in which the base 24 has acontinuous surface supporting the insulating layer 23.

The sensor 30 illustrated in FIG. 2( c) has a “suspended” type ofstructure in which the base 34 has a frame-type shape with a centralopening 37 a and the sensing layer 32 and its insulating layer 33 aresuspended over the opening.

Typically, the measurements obtained from a single semi-conductingmetal-oxide gas sensor element on its own are insufficient to enable agas to be identified with a sufficient degree of certainty, because theselectivity of such sensor elements tends to be low. Accordingly,conventionally these sensing elements are used in arrays of multiplesensing elements disposed side-by-side, and each element in the arrayhas a different material forming its sensing layer. The set ofmeasurements obtained from the whole array forms a cloud of data pointswhich can be processed using statistical techniques so as to determinewhether or not a given gas is present and/or to determine what is thecomposition of the gas mixture that has been presented to the array. Theset of measurements can be considered to represent a kind of fingerprintthat is characteristic of the nature of the gaseous species present inthe gas under analysis and their concentrations.

The selectivity and/or the detection-accuracy of an array ofsemi-conducting metal-oxide gas sensing elements can be improved byincreasing the number of data points used in the statistical processing,for example by deriving multiple measurements from each sensing elementof the array when it is exposed to a given gas sample. This amounts toobtaining a more detailed fingerprint representative of the gas sample.Various techniques are known for obtaining multiple measurements fromeach sensing element in an array of semi-conducting metal-oxide gassensing elements and, in general, they involve changing the operatingconditions applicable when the various measurements are taken, forexample: by measuring the impedance of each sensor element at more thanone operating temperature and/or when different profiles of changingtemperatures are applied to the sensing layer, by sampling the sensinglayer's impedance at different times during its exposure to the gassample, by measuring the sensing layer's impedance with or withoutsimultaneous exposure to ultraviolet light, etc.

Although micro-sensors using semi-conducting metal-oxide gas-sensingtechnology and having a micro-hotplate architecture have been developed,addressing demands for small size and speed of measurement, there is acontinuing need to improve the accuracy of the results produced by suchmicro-sensors.

Other micro-sensors have been developed using chemoresistor technology,notably microsensors using conducting polymers. It is desirable toimprove the accuracy of the results produced by these sensors also.

SUMMARY OF THE INVENTION

The preferred embodiments of the present invention provide achemoresistor type gas sensor having a multi-storey structure whichgathers different measurements from usual for a gas sample, enabling gascomposition and/or identity to be determined based on a new set of datapoints.

The preferred embodiments of the present invention provide achemoresistor type gas sensor having improved performance, notablyincreased accuracy in detecting target gases and improved accuracy indetermining the composition of a gas sample (after an appropriatelearning phase).

More particularly, the present invention provides a gas sensor asdefined in claim 1 appended hereto, and a method of operating a gassensor as defined in claim 12 appended hereto.

In the gas sensors according to the present invention, the adoption of amulti-storey architecture provides a new avenue for generatingmeasurements that can be used for the characterization of the gas underanalysis. At least one first storey in the gas sensor comprises asemi-conducting metal-oxide or conducting polymer gas sensing elementwhich is exposed to the gas under analysis after that gas has alreadybeen in contact with a second storey in the sensor. The second storeycarries a layer of material which, when activated, changes the characterof the gas sample to which it is exposed. The precise composition of thegas contacting the sensing layer of the first storey can be changed byselectively activating the layer of selectively-activatable material ofthe second storey. By measuring the response of the gas sensing elementof the first storey at times when the selectively-activatable materialof the second storey is activated, different data points (measurements)can be gathered compared to the case of using a conventionalsingle-storey architecture consisting of the first storey alone. Thisprovides a new approach for generating the set of data points that canbe included in the statistical processing which characterizes the gasundergoing analysis. Accordingly, an additional degree of freedom isprovided when designing gas sensors adapted to particular applications.

The multi-storey architecture according to the invention can beexploited not only to generate different measurements compared to thosegathered using a conventional single-storey architecture but also toincrease the number of measurements that are gathered, thereby improvingthe accuracy of the results produced by the gas sensor.

Gas sensors embodying the invention can ensure that a satisfactorynumber of measurements are generated, for use in the process ofanalysing/characterising the gas sample, by varying operating conditionsat times when measurements are taken applying techniques based on thosethat have been implemented in prior art sensors, notably by takingmeasurements: at times when the semi-conducting metal-oxide (orconducting polymer) sensing layer and/or the selectively-activatablematerial layer is held at different temperatures and/or is subjected todifferent heating profiles, at times when the semi-conductingmetal-oxide sensing layer (or conducting polymer) and/or theselectively-activatable material layer is exposed to UV radiation and attimes when it is not exposed to such radiation, etc. Moreover, asexplained below certain additional, new techniques are made possible bythe use of a multi-storey architecture.

In certain preferred embodiments of the invention, two or morechemoresistor type gas-sensing elements are stacked to form amulti-storey structure, and the gas undergoing analysis passes throughthe different storeys successively, so that the composition of the gasreaching a subsequent storey is influenced by the presence and operationof the gas sensing layer(s) of the preceding storey(s). This stackedstructure provides the possibility of producing a rich set ofmeasurements, during exposure of the gas sensor to a gas sample, in avariety of ways including but not limited to:

-   -   a) selecting particular combinations of materials for the        sensing layers of the various storeys in the overall gas sensor        device,    -   b) taking measurements when particular combinations of sensing        layers in the stack are activated (and the others deactivated),        and    -   c) measuring the time difference between the time when a sensing        layer at one position in the stack undergoes a particular change        in impedance and the time when a sensing layer at a different        position in the stack undergoes an associated change in        impedance.

The particular combination of materials used in the different sensinglayers can be tailored to the particular target application, forexample: it can be tailored to the expected composition of the gas thatis likely to be encountered, optimized for enabling detection of (ormeasurement of the concentration of) a specific target gaseous species,etc.

In certain embodiments of the invention the selectively-activatablematerial of the second storey does not form part of a gas sensingelement, that is, there is no measurement transducer associated withthis material. In such embodiments it could be considered that thestorey bearing the selectively-activatable material is a type of activefilter which can be switched on or off, as desired. In such embodiments,various different arrangements are possible for positioning theselectively-activatable material of the second storey relative to thesensing layer of the gas-sensing element first storey. In one possiblearrangement a chamber can be defined between the first and secondstoreys and the selectively-activatable material of the second storey aswell as the sensing layer of the first storey can both be located inthis chamber, disposed on opposing walls. In another possiblearrangement, a membrane/substrate is located in-between theselectively-activatable material of the second storey and a spacecontacting the sensing layer of the first storey and openings areprovided in this membrane/substrate so as to enable gas to pass from thesecond storey to the first storey.

The selectivity and sensitivity of gas sensors embodying the invention,and more generally of gas sensors of chemoresistor type, can beincreased by applying pulsed UV radiation to at least one sensing layerin the sensor (instead of constant UV exposure) and ensuring that theproperties of the UV pulses are adapted to the material forming thesensing layer, to the target gas to be detected and to the operatingconditions. In a second aspect, the present invention provides a gassensor as defined in claim 18 appended hereto, and a method of operatinga gas sensor as defined in claim 19 appended hereto.

Measurement data useful to characterise a gas can be obtained byanalysis of transient effects that are observable in the response of agas sensor when the sensor is being exposed to a gas sample and UVillumination is switched on. In a third aspect, the present inventionprovides a gas sensor as defined in claim 20 appended hereto, and amethod of operating a gas sensor as defined in claim 21 appended hereto.

In certain embodiments of the invention it is advantageous to use amicro-hotplate structure to support sensing layers of the gas sensingelements, so as to obtain rapid heating and cleaning of the gas sensinglayers.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features, advantages and applications of the presentinvention will become more apparent from the following description ofpreferred embodiments thereof, given by way of non-limiting examples,and the accompanying drawings, in which:

FIG. 1 is a diagram indicating schematically, in cross-section, thegeneral structure of a semi-conducting metal-oxide gas sensor;

FIG. 2 shows diagrams illustrating the general structure ofsemi-conducting metal-oxide gas sensors having a micro-hotplatearchitecture, in which:

FIG. 2( a) is a cross-sectional view illustrating the overall structureof a micro-hotplate type architecture,

FIG. 2( b) illustrates a “closed” type of micro-hotplate architecture,and

FIG. 2( c) illustrates a “suspended” type of micro-hotplatearchitecture;

FIG. 3 schematically illustrates the stacking of gas sensing elements ofchemoresistor type to form a multi-storey gas sensor according to afirst preferred embodiment of the invention;

FIG. 4 is an exploded view of a portion of a gas sensing element shownin FIG. 3;

FIG. 5 illustrates a multi-storey gas sensor according to the firstpreferred embodiment of the invention;

FIG. 6 illustrates a first approach to mounting gas sensing elements inseries as in the first embodiment of the invention;

FIG. 7 illustrates a second approach to mounting gas sensor elements inseries as in the first embodiment of the invention;

FIG. 8 illustrates a gas sensor having a multi-storey structureincluding stacked arrays of gas sensing elements;

FIG. 9 illustrates a compact gas micro-sensor device incorporating gassensing elements arranged in series as in the first preferred embodimentof the invention;

FIG. 10 illustrates a multi-storey gas sensor in which two gas sensingelements are mounted in series in a back-to-back arrangement on a singlesubstrate;

FIG. 11 illustrates a multi-storey gas sensor in which successive gassensing elements are spaced from each other;

FIG. 12 is a schematic representation of a second embodiment of theinvention, in which a chemoresistor type gas sensing element is arrangedin series with a material that can be activated to change the characterof the gas sensed by the gas sensing element;

FIG. 13 is a schematic representation of a first variant of the secondembodiment;

FIG. 14 is a schematic representation of a second variant of the secondembodiment;

FIG. 15 is a schematic representation of a third variant of the secondembodiment;

FIGS. 16A and 16B illustrate a new technique for increasing gas sensorsensitivity and selectivity by controlling the time of exposure of asensing layer to ultraviolet light, in which:

FIG. 16A shows an example of UV pulses suitable for use in the newtechnique, and

FIG. 16B shows a test arrangement used to measure the effect of appliedUV pulses;

FIGS. 17A to 17C show experimental results obtained using the testarrangement of FIG. 16B, in which

FIG. 17A shows results obtained when a semiconducting metal oxide gassensor was exposed to air+H₂S without application of UV, with constantexposure to UV, and with exposure to UV pulses of controlled duration,

FIG. 17B shows results obtained when a gas sensor was exposed to air+NH₃without application of UV, with constant exposure to UV, and withexposure to UV pulses of controlled duration; and

FIG. 17C compares the results obtained in FIGS. 17A and 17B in the caseof gas samples at a concentration of 1 part per million;

FIG. 18 illustrates transient effects seen in the response of a sensinglayer at onset of exposure to ultraviolet light, in which:

FIG. 18A shows sensor response during testing of a sample containingH₂S, and

FIG. 18B shows sensor response during testing of a sample containingNH₃; and

FIG. 19 illustrates various other architectures according to the firstaspect of the invention, in which two sensing elements are mounted inseries.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

Certain presently-preferred embodiments of the invention will now bedescribed with reference to FIGS. 3 to 19.

A first, currently-preferred embodiment of the invention is illustratedin FIGS. 3 to 5. According to the first preferred embodiment of theinvention a multi-storey chemoresistor type gas sensor is constructed bystacking two (or more) gas sensing elements CH1, CH2 to form amulti-storey structure. FIG. 3 illustrates how the gas-sensing elementsCH1, CH2 are arranged relative to one another. FIG. 4 is an explodedperspective view of the structure of each of the gas-sensing elementsCH1, CH2 (omitting the base substrate), and FIG. 5 illustrates anoverall gas sensor structure that includes the gas-sensing elements CH1,CH2 stacked as illustrated in FIG. 3 and indicates gas flow through thesensor.

FIGS. 3 to 5 illustrate the stacking of two gas sensing elements but itis to be understood that the invention can be implemented also inembodiments in which three or more gas sensing elements are stacked.

As illustrated in FIGS. 3 and 4, each of the sensing elements CH1, CH2includes a sensing layer 52 made of a semi-conducting metal oxide,supported on a membrane structure 53. The sensing layers may be made ofvarious materials including, but not limited to SnO₂, In₂O₃, ZnO, RuO₂,WO₃, and AB₂O₄ (spinel type oxides); catalytic materials can also beused (typically mixed with the oxides), such as platinum, rhodium, gold,etc.; and materials can be used which have both sensing and catalyticproperties e.g. titanium oxide. Alternatively, if the sensing layer ismade of a conducting polymer then it may be made of various materialsincluding, but not limited to, polyaniline, polypyrrole, polythiophene,polyacetylene, poly(phenyl vinlene), andpoly(3,4-ethylene-dioxythiphene), with any desired doping.

The present invention is not particularly limited with respect to thetechniques used for deposition of the sensing material (and anycatalytic material). As is well-known, the nature of the surface of thedeposited sensing/catalytic material influences the efficiency of thesensor; nano-particles, and porous surfaces produced by physical vapourdeposition (PVD), yield good efficiency. In general, the depositiontechnique will be adapted to the particular material being deposited,bearing in mind efficiency considerations. The thickness of the layer 52will vary depending on the deposition technique and, typically, will be100-1000 nm when PVD is used, and 10-100 μm otherwise (although thesevalues can be varied).

In the example illustrated in FIGS. 3 and 4 the membrane structureconsists of three thin layers 53 ₁, 53 ₂, 53 ₃ of insulating material(for example SiO₂, or Si₃N₄, or SiO_(x)N_(y), or SiN_(x)). The thinlayers 53 ₁, 53 ₂ sandwich a heater (described below), and serve toinsulate this heater from other components. The layer 53 ₃ functions asa membrane to support the overlying layers. Stresses in this multi-layermembrane structure can be reduced by forming the layers from differentmaterials. In this example, layer 53 ₁ is made of SiO₂, layer 53 ₂ ismade of SiN_(x) and layer 53 ₃ is made of SiO₂.

The membrane structure 53 is mounted on a base substrate 54 which isrelatively thick at the edges but has a recess 57 so as to provide amicro-hotplate structure. In the example shown in FIG. 3 the recess 57takes the form of an opening through the base substrate. However, themicro-hotplate structure is still of the “closed” type illustrated inFIG. 2( a) because the membrane structure 53 covers the opening in thebase substrate 54. Typically the base substrate 54 is made from asilicon wafer because Si wafers can be machined with high precisionusing standard semiconductor manufacturing processes.

As illustrated in FIG. 3, holes 58 are provided through the membranestructure 53. Alternatively, the layers making up the membrane structure53 may be porous. In this example the holes/pores 58 have a diameter of10 μm, but the invention is not limited to this value.

Because the gas-sensing elements CH1, CH2 have a closed type ofmicro-hotplate structure, and because holes 58 (or pores) are providedin registration with the sensing layers 52, gas passing through the gassensor 50 traverses each sensing layer 52, maximising the contact thateach sensing layer 52 has with the gas sample. Firstly, this tends tomaximise any effect that the gas produces on the electrical propertiesof this sensing layer 52 and, secondly, this tends to increase changesin the gas that flows to the subsequent storey.

As indicated in the previous paragraph it is advantageous to locate theholes 58 in the active area of the sensor (i.e. in registration with thesensing layer 52). However, the position of the holes 58 can be varied.If all the holes 58 are outside the active area then the gas passing tothe subsequent storey will undergo less modification during its passagethrough the current storey, but it will still be possible to measure thetime taken for the gas to diffuse from this storey to the next.

In each of the sensing elements CH1, CH2, measurement electrodes 55 areprovided in contact with the respective sensing layer 52 so as to detectchanges in the electrical properties of the sensing layer when it isexposed to a gas. The particular changes that take place depend on thenature of the material forming the semi-conducting metal oxide and onthe gaseous species present in the gas sample but, in general, consistof oxidation and/or reduction reactions changing the impedance of thesensing layer. As indicated above, in general it is necessary to heatthe sensing layer in order for appreciable adsorption (andoxidation/reduction) to take place. Accordingly a heater 56 is providedin-between the insulating layers 53 ₁, 53 ₂. The heater itself can alsobe used as a temperature sensor monitoring a change of the resistance.Although not shown in the figures, each gas sensing element CH1, CH2also includes a different temperature sensor so as to be able to monitorthe temperature attained by the sensing layer 52.

In this example the measurement electrodes are made of Pt, with anunderlying Ti adhesion layer, and take the form of two interlockedconductor elements having portions taking a generally circular shape (asshown in FIG. 4). In practice the measurement electrodes 55 have agreater number of interlocking circular portions than represented inFIG. 4. In this example each measurement electrode is 0.2 μm thick and10 μm wide, and the measurement electrodes are spaced apart from oneanother by 10 μm. The precise positioning and shape of the measurementelectrodes 55 can be varied. However, in this example the measurementelectrodes 55 are positioned and shaped so as to be well-located fordetecting the expected change in electrical properties of the sensinglayer 52.

In a similar way, in this example the heater 56 takes the form of agenerally circular element which underlies the sensing layer 52. In thisexample the heater 56 is made of Ti/Pt like the measurement electrodes,but in the case of the heater 56 the Ti/Pt forms a heater patternincluding conductive traces which bend back on themselves to form aseries of nested loops. Typically the conductive traces of the heaterpattern are 0.2 μm thick and 20 μm wide. Materials other than Ti/Pt, forexample multilayers of refractory conductors (Mo, Ta, W, . . . ), butalso polysilicon, may be used for the heater 56.

The precise positioning of the heater 56 and temperature sensor can bevaried. However, the transfer of heat from the heater 56 to the sensinglayer 52 is particularly efficient when the heater is provided inregistration with the position of the sensing layer, and shaped, asshown in FIGS. 3 and 4. In this example the heater 56 is separated fromthe sensing layer 52 by the membrane 53 ₁ so as to ensure electricisolation of the heater 56 from the measurement electrodes 55.

In one example using measurement electrodes 55 and a heater 56 havingthe configuration illustrated in FIG. 4, the insulating layers 53 ₁, 53₂ are about 0.5 μm thick layers of SiO₂ and SiNx, the other insulatorlayer 53 ₃ is about 0.8 μm thick SiO₂ layer and the substrate 54 is asilicon wafer of 300˜500 μm thick at the edges. In this example, asensing layer 52 made of ZnO can be brought up to a temperature of 500°C. very rapidly (notably in 30 milliseconds).

Any convenient techniques can be used for fabricating the sensingelements CH1, CH2.

As shown in FIG. 3, the sensing elements CH1, CH2 are stacked relativeto one another. In general, the sensing layer 52 is porous because ithas a grain-based structure or is made of nano-particles, nano-rods,nano-wires or nano-tubes. In preferred embodiments of the invention thesensing layer 52 has a nano-particle structure because the ratio ofsurface area to volume is high for such a structure, providing a largesurface area on which chemical reactions can occur with the gas undertest. When the sensing layer 52 of the sensing element CH1 (or of thesensing element CH2) is exposed to a gas sample the gas will penetrateinto and through the sensing layer 52 because the sensing layer isporous.

In the first preferred embodiment of the invention, the layers 53 ₁, 53₂, and 53 ₃ are traversed by holes 58 so that the gas penetrating thesensing layer 52 passes all the way through the relevant gas sensingelement. Accordingly, gas will pass through the sensing layer 52 of onesensing element (say CH2) before reaching the sensing layer 52 of thesubsequent gas-sensing element (say CH1). The holes 58 may be made bystandard processes used in semiconductor manufacture (for example usingphotolithography and reactive ion etching, etc.) In a variant structurethe underlying layers 53 ₁, 53 ₂, and 53 ₃ are porous and it is then notnecessary to provide the holes 58. Incidentally, in a case where therecess 57 is spanned by a thin portion of the substrate 54, underlyingthe membrane structure 53, holes 58 can be provided in that substrateportion also, or it can be made of a porous material.

The gas passing through one of the gas-sensing elements (e.g. CH2) ismodified, notably dependent on the type of oxide or conducting polymerused in the sensing layer, the temperature of the sensing layer, effectsof exposure to UV radiation (if any) and the time-profile of thetemperature that is applied to the sensing layer. Accordingly, thesignal measured by the measurement electrodes 55 of a subsequentgas-sensing element (e.g. CH1) depend not only on the nature of theoxide or conducting polymer in the sensing layer of this gas sensingelement CH1, and its operating conditions (temperature, exposure to anyUV radiation, time-variation in applied temperature, frequency of thevoltage, etc.) but also on the nature of the oxide or conducting polymerin the gas-sensing element of the preceding storey and its operatingconditions.

FIG. 5 illustrates the overall structure of a gas sensor based on thestacked sensors of FIG. 3. The gas flow through this sensor isillustrated using an arrow marked using dashed lines. As shown in FIG. 5the gas-sensing elements CH1, CH2 are stacked on a base 60 inside ahousing 65 having an inlet (for receiving gas to be analysed) and anoutlet. The base 60 may be made, for example, of glass and the housing65 made of PTFE, although the invention is not limited to thesematerials. Depending on the number of gas sensing elements to betraversed by the gas sample under analysis, it may be necessary ordesired to use a pump or the like to force gas circulation through thegas sensor. However, as explained below, a useful measurement can beproduced by allowing the gas sample to simply diffuse through the gassensor. It is believed that simple diffusion, without forcedcirculation, should be sufficient to achieve satisfactory measurementsin an acceptable time frame in a gas sensor employing two or threestacked gas-sensing elements.

According to the first preferred embodiment of the invention,gas-sensing elements can be stacked in numerous differentconfigurations. A few of the possible configurations are illustrated inFIGS. 6 to 11.

FIG. 6 shows a configuration in which four gas-sensing elements arearranged in series (stacked) between a gas inlet and a gas outlet, and alight-emitting diode is provided in the housing so as to be able toilluminate the gas-sensing elements with ultraviolet light. Leads can beseen passing through the side wall of the housing for connecting theheaters and measurement electrodes of the various gas-sensing elementsto external circuitry (and, in practice, additional leads could beprovided, if required, for temperature sensors).

In the FIG. 6 configuration, the LED may be arranged to illuminate allof the sensing elements in the housing, notably by supporting thesensing layers of the various storeys on membranes/insulating layersthat allow UV to pass (e.g. very thin layers, notably layers up toaround 2 μm thick). Alternatively, if it is desired to illuminate some,but not all, of the sensors with UV light then UV barrier layers may beprovided at appropriate locations in the housing.

FIG. 7 illustrates two stacked gas-sensing elements, and a gasconnector, mounted on a standard TO5 type support for connect-and-plugsemiconductor devices.

FIG. 8 illustrates a multi-storey architecture in which arrays ofgas-sensing elements are stacked. In this example one 2×2 array ofsemi-conducting metal-oxide gas sensors is stacked on another 2×2 array.However, it is to be understood that the invention is not limited withregard to the dimensions of the stacked arrays nor with respect to thenumber of arrays included in the stack. Moreover, it is not excluded tovary the dimensions of the arrays from one storey to the next.

FIG. 9 illustrates a compact device incorporating a gas sensor accordingto the first embodiment of the invention as well as circuitry to drivethe gas-sensing elements and process their output. This compact devicetakes the form of a capsule roughly 1 cm by 2.5 cm in size yet includingmultiple gas-sensing elements (notably, two to seven stacked gas-sensingelements).

FIG. 10 illustrates a device in which two gas sensing elements aremounted n series using a single supporting substrate 54. It isadvantageous to use different materials for the sensing layers 52, 52′of the two gas sensing elements.

FIG. 11 illustrates a gas sensor device in which the series-connectedgas sensing elements of the multi-storey structure are spaced from eachother, possibly with intervening elements (indicated by the box indashed lines in FIG. 11). This architecture differs from known devices(containing a series of chambers through which gas passes, withdifferent sensors in each chamber), by virtue of the fact that the gaspassing through the device is obliged to pass through the sensinglayer/membrane of the upstream gas sensing element CH1 before reachingthe downstream gas sensing element CH2 and, thus, the gas is modifiedbefore it reaches the downstream sensing element.

Although FIG. 11 shows the second gas sensing element CH2 oriented in amirror-image orientation relative to the first gas sensing element CH1,it will be understood that both elements may be arranged in the sameorientation in the gas flow path. Moreover, it will be understood thatthree or more gas sensing elements may be interconnected in the mannershown in FIG. 11.

Although not shown in FIG. 3 to 8, 10 or 11, the measurement electrodes55, heaters 56 and temperature sensors in the gas sensors according tothe first preferred embodiment of the invention all are connected to (orare adapted for connection to) other circuitry, notably for supplyingcurrent and taking measurements using well-known techniques that do notrequire description here.

It is worthwhile to mention the following features specific to thepresent invention.

a) How to Arrange Various Sensing Materials “Vertically” (i.e. in theVarious Storeys)

In general, the number and choice of materials, as well as theirrelative positioning on the different storeys of a multi-storey deviceaccording to the invention is designed so as to increase thediscriminant ability of the overall sensor device in a manner dependenton the specific target application. Numerous configurations will readilyoccur to the skilled person based on his common general knowledge,notably in view of the reactivities of different gases with differentsemiconductor metal oxide and conducting polymer materials. For thepurposes of illustration, an example is discussed below regarding howdiscrimination of a target gas in the presence of an interference gascan be made easier by virtue of the choice of a combination of materialsto use in the sensing layers of the different storeys in a multi-storeysensor according to the invention.

All gases have different reactivities with conducting polymers and metaloxide gas sensing materials such as ZnO, SnO2, In₂O₃, and so on. It isvery difficult to discriminate a gas which has lower reactivity with aspecific material when the target gas is in an interference gas whichhas higher reactivity. For example, to distinguish NH₃ is very difficultin an H₂S atmosphere because H₂S is more reactive in comparison withNH₃. According to previous research, the bond energy of H—SH is 381KJ/mol, and the bond energy of H—NH₂ is 450 KJ/mol. Additionally, twoNH₃ molecules release three free electrons through a reaction withadsorbed oxygen on the surface of ZnO. On the other hand, one H₂Smolecule releases three free electrons through the reaction. As aresult, H₂S is more reactive than NH₃ with a ZnO sensing material.However, as described below, NH₃ is selectively detectable using amulti-storey architecture according to the invention.

For example, suppose that there are two kinds of sensing material. One,Material A, shows about 90% reactivity and 10% reactivity for H₂S andNH₃, individually. And the other, Material B, shows about 60% reactivityand 40% reactivity for H₂S and NH₃, individually. The expression“reactivity” mentioned here denotes a parameter quantifying how manymolecules can decompose into inactive gases on the surface of metaloxide sensing materials. For example, NH₃ can decompose into H₂O and N₂on the surface of ZnO and H₂S can decompose into H₂O and SO₂ on thesurface. It is impossible to generate a larger measurement signal forNH₃ using these two materials, A and B, in an H₂S atmosphere. However,it is possible to generate a larger measurement signal for NH₃ if thesensing materials are arranged vertically, that is, in subsequentstoreys of a multi-storey architecture according to the invention.

Suppose that material A forms the sensing layer in the top storey,through which gas enters, and material B forms the sensing layer in thebottom storey that receives gas after passage through the top storey. Inthis case, gas can get to the surface of material B only throughmaterial A. Suppose that 1000 molecules each of NH₃ and H₂S pass throughthe materials. While the 1000H₂S molecules pass through Material A, 900of the molecules decompose into H₂O and SO₂. On the other hand, in thecase of NH₃ under the same conditions, only 100 molecules decompose intoH₂O and N₂. This means that a greater number of NH₃ molecules can reachand react with Material B in comparison with the number of H₂Smolecules. In the above example, 900NH₃ molecules can get to Material B,but only 100H₂S molecules can get to Material B. Accordingly, themeasurement signal of NH₃ increases relatively in comparison with thesignal of H₂S. This is a basic example of how materials may be selectedto ensure that the gas sensor device has good selectivity for a gaswhich has lower reactivity than an interference gas. As mentioned above,there can be various combinations for the sensing materials and theirrelative disposition depending on the specific application.

b) How to Use Selective Activation of Various Sensors of theMulti-Storey Architecture.

One of the advantages of the multi-storey architecture provided by theinvention is that this architecture makes it possible to make use (forgas discrimination) of the difference of gas diffusivity through sensorswhich are arranged in series. The diffusivity through each storey isdependent on the sensing-layer material itself and its operatingtemperature. So, it is important what kinds of materials are used andalso important what is the operating temperature. This means that theoperating sequence is very important to control the diffusivity. Themulti-storey architecture can be used with various operating sequenceson the temperature. To remove an interference gas for target gases,operating temperature should be well controlled. But a profile of thesequence can be determined experimentally for a specific application.

c) Measurement of Time Difference Between Reactions of SuccessiveSensing Elements:

In an embodiment including at least two stacked gas sensing elements,one of the additional techniques that can be implemented for increasingthe size of the cloud of data points to be analysed, and which derivesfrom the use of a multi-storey architecture, consists in measuring thetime difference between a moment when the sensing layer of one storey inthe architecture reacts to the gas undergoing analysis and a moment whenthe sensing layer in a subsequent storey (e.g. the next storey in thedirection of gas flow) has a comparable reaction to the gas. When theflow of gas is not being forced, this time difference can becharacteristic of the rate of diffusion of the gas through the sensinglayer of the “one” storey and, in view of the fact that rates ofdiffusion of various gaseous species through different materials areknown, can serve as an additional parameter helping to identify a gas inthe sample undergoing analysis.

The waveforms produced by the measurement electrodes have variouscharacteristic points, for example points where the gradient changessharply. The particular characteristic point that is best suited toserve as the reference for measuring the above-described time differencetends to vary depending on the particular application. In one example, atime t1 is measured corresponding to the moment when an inflection pointfirst appears in the signal output by the measuring electrodes of anupper storey that is first in the gas flow path (this momentcorresponding to the onset of a reaction between the sensing layer ofthis storey and the gas undergoing analysis) and a time t2 is measuredcorresponding to the moment when a corresponding inflection point firstappears in the signal output by the measuring electrodes of thesubsequent storey in the gas flow path (this moment corresponding to theonset of a reaction between the sensing layer of this subsequent storeyand the gas undergoing analysis). The time difference t2−t1 ischaracteristic of the membrane and of the gas and, notably, isinfluenced by the sensing material 52 in at least the earlier storey inthe gas flow path, the size of holes/pores in the layers 53 ₁-53 ₃ andthe distance between the two sensing layers. The gas can be identifiedusing Fick's Law and known values of diffusion coefficients fordifferent gases.

In a case where the gas under analysis includes more than one speciesthe response of the measurement electrodes will often include respectivedifferent features (notably, different points of inflection) that arecharacteristic of the sensing layer's reaction to these differentgaseous species. Accordingly, in this case data can be generated foreach gaseous species by measuring the time interval between thecorresponding feature in the response of the measurement electrodes ofone storey and the equivalent feature in the response of the measurementelectrodes of the subsequent storey.

d) How to Produce a Rich Set of Data Points

The first embodiment of the invention enables a rich set of data pointsto be generated rapidly using a small structure. The number and varietyof data points in the set of measurements used for characterizing a gasmay be enhanced in a variety of ways, notably:

-   -   by increasing the number of gas-sensing elements,    -   by increasing the number of different materials forming the        sensing layers,    -   by varying the operating conditions applicable to the various        sensing layers notably: the operating temperatures,        exposure/non-exposure to UV light, applying different        time-varying profiles of the operating temperatures (notably        applying a profile which involves short intervals at different        temperatures and measuring the short-term reactions of the        sensing layer), applying a pulsed potential difference to the        measuring electrodes and varying the pulse frequency, etc.)

The present invention is not particularly limited with regard to theimplementation of a process for generating data points by varying theoperating conditions of the various sensing layers in a multi-storeydevice according to the invention. Different choices can be maderegarding: whether to alter the operating conditions of a single sensor,or multiple sensors, at the same time; whether to measure output signalsfrom all sensors, or a sub-set of the sensors, when operating conditionsof particular sensors are varied; whether to take discrete measurements(e.g. one or more sets of discrete measurements for each configurationof the operating conditions applied to the set of sensors) or continuousmeasurements as the operating conditions are varied, etc. For example,plural data points may be generated by simultaneously applying differenttemperature profiles to each of the sensors in the device and measuringthe outputs from the measurement electrodes of all storeys. In general,the different operating conditions applied to the various sensing layerscan be tailored to the specific application, in order to optimize thediscrimination performance of the device.

A further measurement can be generated in embodiments of the presentinvention by paying attention to the energy that is needed to maintainthe sensing layer (or the selectively-activatable material) at aparticular temperature. More particularly, some of the reactions thatoccur when a gas contacts the sensing layer (or theselectively-activatable material) may be endothermic whereas others maybe exothermic. The heat changes during such reactions alter the amountof energy that is needed by the heater in order to keep the relevantsensing layer (or selectively-activatable material) at the nominaloperating temperature. This alteration can be detected and used as anadditional indication of the properties of the gas sample beinganalysed. Moreover, this technique can be applied in the secondembodiment of the invention described below.

Various interesting new techniques based on the use of pulsed UV lightcan be applied in order to increase the sensitivity and selectivity ofthe above-described gas sensors and, indeed, to increase the sensitivityand selectivity of chemoresistor gas sensors in general (i.e. evenwithout using a multi-storey architecture). These new techniques willnow be described with reference to FIGS. 16-18.

It has already been proposed to expose semiconducting metal oxide gassensors to ultraviolet light continuously during a period whenmeasurements are being taken. Further, it has been proposed to apply theUV light in pulses, in order to reduce energy consumption. However, thepresent inventors have discovered that the sensitivity and selectivityof a semiconducting metal-oxide gas sensor in relation to differentgases can be increased dramatically by applying pulsed UV, notcontinuous UV, and by tailoring the pulse properties to the specificgases, taking into account the operating temperature and the nature ofthe sensing material. It is believed that this phenomenon arises for thereasons explained below.

Gas molecules have different structures and different binding energiesfor the various bonds in those molecules. For example, in a molecule ofH₂S, the bond energy of the H—SH bond is 381 kJ/mol, and in a moleculeof NH₃ the bond energy of H—NH₂ is 450 kJ/mol. When pulses of UV lightare applied to a semiconducting metal oxide sensor during exposure to agas, the duration of the pulses required to activate the moleculesvaries with the gas species. For a given gas there appears to be anoptimal duration of UV illumination in order to maximise the effect thatcan be observed in the response of the sensing material as detected viathe measurement electrodes.

Experiments were conducted using a single semiconducting metal oxide gassensor having a ZnO sensing layer. Effectively, the specificcharacteristic of metal oxides such as ZnO, which is photosensitive, isemployed. The sensor was exposed in turn to a test atmosphere consistingof clean dry air (relative humidity 0%), and to a gaseous atmospherecontaining varying amounts of H₂S, under different conditions. In eachcase the sensing material was exposed to a continuous flow of gasthrough the sensor at a rate of 200 cm³ per minute.

In a first set of experiments the ZnO layer was exposed to pulses of UVlight of wavelength 385 nm having the general form illustrated in FIG.16A while being exposed to the test gases. In each case the response ofthe ZnO sensing layer was measured 90 ms after the start of each UVpulse (as indicated by the arrows M in FIG. 16A). In a second set ofexperiments the ZnO layer was exposed to continuous UV illumination atthe same 385 nm wavelength while being exposed to the same set of testgases. The experiments were repeated a third time, without UV exposure.

The wavelength of the UV pulses was set at 385 nm in view of the bandgap in ZnO. The energy necessary for a charge carrier to cross the bandgap in ZnO is 3.3 eV at 25° C., which corresponds to a UV wavelength of376 nm. Based on theoretical calculations it would be expected that UVwavelengths of 376 nm or below should be used for the UV pulses in orderto supply sufficient energy for crossing of the band gap in ZnO; and,indeed, successful experiments were performed using UV pulses having awavelength of 365 nm. However, in practice, it was found that usefuleffects were still produced using UV pulses at 385 nm. In general, it isadvantageous to set the wavelength of the applied UV pulses based on theenergy necessary to cross the band gap in the sensing material inquestion.

During the tests involving pulsed UV, 100 msec after the end of each UVpulse there was a 20 msec period of heating the ZnO sensing material to530° C. (indicated in FIG. 16A by the dashed pulses TP). Comparable 20msec heating periods were applied at comparable times during the testsinvolving continuous UV and no UV. These short bursts of heating improvethe return of the measurement signal to its baseline value.

In these experiments, the response of the ZnO sensing layer was measuredusing a resistance divider structure as illustrated in FIG. 16B. Theresistance Rs illustrated in FIG. 16B represents the resistance of thesensing layer ZnO, the resistance RL represents a reference resistorwhich was connected in series with the ZnO sensing layer. In theseexperiments, RL was 2.74 MOhms, and the illustrated d.c. power supplyprovided a voltage of 3.3 volts. The voltage at a point between the ZnOsensing layer and the reference resistor RL was supplied to aprogrammable gain amplifier which, in turn, supplied the amplifiedsignal to a microcontroller, and measured once per minute. The averageof the last four measurements was calculated, to represent the responseof the ZnO sensing layer, and the response was monitored over a 20minute time period.

Incidentally, it will be understood that FIG. 16B is just one example ofa measurement circuit that may be used to evaluate the resistance of thesensing layer. Various modifications may be made in the circuit of FIG.16B (e.g. the output from the amplifier could be supplied to a low passfilter and then to an analog-to-digital convertor), or indeed othercircuit arrangements could be used.

The sensitivity of the sensor to H₂S was quantified by evaluating aparameter Ra/Rg where Ra is the resistance of the ZnO sensing materialduring the exposure to dry air and Rg is the same resistance but duringexposure to the atmosphere containing H₂S. The experiments wereperformed using different gas samples containing H₂S at concentrationsof 0.1 part per million (ppm), 0.5 ppm and 1.0 ppm, respectively. Theresults are shown in FIG. 17A.

As can be seen from FIG. 17A, the sensitivity of the gas sensor to H₂Sis lowest when there is no exposure to UV light, the sensitivityimproves somewhat (perhaps twofold) when the measurement is taken duringcontinuous exposure to UV, but there is a large improvement insensitivity—roughly 5 to 17 times—when the sensor is illuminated usingthe UV pulses of FIG. 16A having a duration of 100 ms and a duty cycleof 10%.

The above experiments were repeated using a gaseous atmosphere of airwith varying amounts of NH₃ (using NH₃ concentrations of 1, 5 and 10ppm) but otherwise leaving the conditions unchanged. The results areshown in FIG. 17B.

As can be seen from FIG. 17B, the sensitivity of the gas sensor to NH₃is increased when the sensor is illuminated using the UV pulses of FIG.16A. However, the increase in sensitivity of the gas sensor to H₂S whenthe UV pulses are applied is significantly greater than the sensitivityincrease that is obtained in the case of NH₃.

FIG. 17C illustrates the relationship between the increase insensitivity that was observed for gas samples containing H₂S at aconcentration of 1.0 part per million (ppm) and that observed for gassamples containing NH₃ at a concentration 1.0 ppm. It can be seen thatthe sensitivity increase observed for H₂S is of the order of 1000 timesthe sensitivity increase observed for NH₃. The skilled person willreadily appreciate that the selectivity of the gas sensor for H₂S (i.e.its responsiveness preferentially to H₂S compared to other gases) isdramatically increased by applying the UV pulses.

It is considered that application of UV pulses of selected wavelengthwhose duration is suitably set dependent on a target gas (and,advantageously, also set taking into operating conditions) will producean improvement in gas-sensor sensitivity even when gaseous speciesdifferent from H₂S and NH₃ are being detected and when the sensingmaterial is different from ZnO, provided that the activation energy ofthe target gaseous species in respect to the chosen sensing material atthe selected operating temperature is in a range suitable to be providedby pulses at UV wavelengths. Suitable sensing materials include othersemiconducting metal oxide materials (e.g. TiO₂, or SnO₂ at hightemperature, etc.) and semiconducting chalcogenides (e.g. CdS, ZnS,etc.), as well as selected conducting polymers. The optimal durationand/or duty cycle of the UV pulses depends on the target gaseousspecies, as well as on the selected sensing material, on the wavelengthof the applied UV pulses and on the operating temperature.

This technique of applying specifically-designed UV pulses to improvegas sensor sensitivity and selectivity can be employed so as to enable aparticular gas sensor to be used to discriminate plural gases. Moreparticularly, a gas sensor can be provided with a control moduleconfigured to set the duration and/or duty cycle of UV pulses applied toa sensing layer in the gas sensor to different values each suitable toincrease the sensitivity of the sensing layer to a respective differentgaseous species (the appropriate value for the duty cycle and operatingtemperature being determined by experiment, taking into account thetarget gaseous species, as well as the selected sensing material and theUV pulse wavelength chosen for the selected sensing material).

Alternatively, or additionally, a gas sensor can be provided with acontrol module configured to apply a measurement protocol adapted toenhance the selectivity of the gas sensor to a target gas. Moreparticularly, the measurement protocol may involve taking measurementsfirst without applied UV and then with applied UV pulses ofpredetermined wavelength and duty ratio (the duty ratio and operatingtemperature being set—based on the results of prior testing for theselected sensing material/UV wavelength—to produce a significantincrease in sensitivity towards the target gas compared to the increasein sensitivity expected for other likely gas species in the samples). Acomparative analysis of the sensor responses observed in the two sets ofmeasurements can be used for detection of the target gas species.Typically, chemometric methods are used to analyse the sensor responsesbut the invention is not particularly limited with regard to thecomparative analysis methods that may be applied.

As mentioned above, the technique according to the second aspect of theinvention, involving application of specially-tailored UV pulses, canproduce a dramatic increase in the selectivity of a gas sensor (of theorder of 1000 times). However, such a dramatic increase in selectivityis not needed in all applications. In some cases it may be sufficient toset the properties of the UV pulses so that a lesser increase inselectivity is obtained, which is still significant in the context ofthe specific application, even if this is not the maximal increase thatcould have been obtained for the given gas species and sensing material.

Although the above-described experiments were performed using a gassensor including a single ZnO sensing layer, it is believed that theobserved improvement in selectivity and sensitivity obtained byapplication of UV pulses is not specific to such gas sensors, but on thecontrary would occur also in other architectures of chemoresistor gassensor. Thus, it is proposed to apply the above-described technique inmulti-storey devices according to the various embodiments of theinvention described in this document and, more generally, in otherchemoresistor-type gas sensors and gas sensors based on conductingpolymers.

When the above-described technique is applied in the multi-storey sensorarchitectures described above, the UV pulses may be applied to one ormore of the storeys in the sensor. When UV pulses whose properties havebeen set in view of improving detection of a specific target gas areapplied to the sensing material (or, more generally to theselectively-activatable material) of a given storey in the sensor, thenthe illuminated material will have a stronger response to the target gasthan would have been the case without UV illumination (or, indeed, UVillumination with other pulses or continuously). Accordingly, in thecase where this storey includes measurement electrodes the strength ofthe signal measured in this storey of the sensor will change. Moreover,the gas that passes from this storey to a subsequent storey in thesensor will have undergone a larger modification by the current storeythan would have been the case if the UV pulses had not been applied.Both of these effects can be used as sources of data to helpcharacterise the gas sample under analysis.

It will be understood that when the above-described technique is appliedin the multi-storey sensor architectures described above, the propertiesof the UV pulses may be set dependent on a target gas species which thegas sensor aims to detect or, if desired, dependent on an interferinggas species that the UV-illuminated storey of the gas sensor is designedto prevent from reaching a subsequent storey. If desired, bothapproaches may be used simultaneously in different storeys of the samegas sensor, although the size of the sensor is liable to increase ifmultiple UV sources are provided.

The present inventors have further realized that the set of measurementsobtainable using gas sensors according to the invention, and indeedother semiconducting gas sensors, can be further enriched by making useof another phenomenon observed when UV pulses are applied to the gassensors. More particularly, the inventors have observed that when the UVpulse is first switched on transient effects can be observed in theresponse of the sensing material and the transient effects are differentwhen the sensing material is being exposed to different gas species.This phenomenon is illustrated by FIG. 18.

The traces shown in FIGS. 18A and 18B were measured during experimentsperformed using a test circuit comparable to that of FIG. 16B exceptthat, in this case, the gas sensor was a two-storey sensor according tothe first embodiment of the invention, in which the upper storeyconsisted of a micro-sensor with holes through the membrane and thelower storey consisted of a micro-sensor without holes in the membrane.In these experiments the two storeys of the micro-sensor both employedsensing layers made of ZnO, the upper storey using ZnO nanoparticles andthe lower storey using ZnO nanorods. A stream of gas was made to flowcontinuously through the sensor at a rate of 200 cm³ per minute: first astream of air (for 2400 seconds), then a gas sample containing H₂S at 1ppm (for 1200 seconds), then air again (for 2400 seconds), followed by agas sample containing NH₃ at 1 ppm (for 1200 seconds), and finally air(for 2400 seconds). The relative humidity of the gas was 50% in eachcase.

During each time period when the gas sensor was exposed to a new gasthere were three measurement phases starting 100, 400 and 700 secondsbefore the changeover to the next gas, and each measurement phase lasted20 seconds. During each measurement phase the sensor was exposed in acontinuous fashion to UV light having a wavelength of 365 nm and thesensor response was measured every 10 milliseconds: the start and end ofUV exposure was synchronised with the start and end of the measurementphase and the first measurement took place 10 milliseconds after thestart of UV exposure. Accordingly, 2000 measurements were made duringeach measurement phase and each period of UV exposure was 20 secondslong.

FIG. 18A illustrates the response of the upper storey of the gas sensorduring a typical one of the measurement phases that was performed whileH₂S was flowing through the sensor. FIG. 18B illustrates the response ofthe upper storey of the gas sensor during a typical one of themeasurement phases that was performed while NH₃ was flowing through thesensor.

It can be seen from FIG. 18A that while the sensing material is beingexposed to H₂S there is a transient spike in the sensor response justafter UV exposure begins, and the sensor resistance at the end of thespike (right-hand side of the spike as shown in FIG. 18A) is lower thanthe value at the start of the increase (left-hand side of the spike).Subsequently, the resistance value increases relatively slowly, whileshowing rapid short-term fluctuations.

In contrast, FIG. 18B shows that while the sensing material is beingexposed to NH₃ the sensor response just after UV exposure begins has asharp drop in resistance, and this fall in resistance continuessmoothly, at a decreasing rate, as time goes on.

It is believed that the transient effects observed in the response ofthe sensing material at the onset of UV exposure are characteristic ofthe respective gas species to which the sensing material is beingexposed, thus providing a tool for detecting specific substances.

One technique for exploiting the above-described phenomenon is, asfollows. During a preliminary learning phase a selected sensing materialis exposed to a set of one or more test substances and the transienteffects observed in the response of the sensing material to therespective test substance(s) when UV exposure begins are measured.Discriminant analysis techniques are then used to determine thedistinctive features of the transient effects that enable the testsubstance(s) to be characterised. During a subsequent measurement phase,the selected sensing material is exposed to a gas sample and, during theperiod of exposure, UV illumination is switched on. The response of thesensing material is measured so that the transient effects can beanalysed and a comparative analysis can be performed with the transienteffects measured for the set of test substances. This enables a specifictest substance to be detected if it is present in the gas sample underanalysis.

In the experiments described above with reference to FIG. 18, the sensorresponse at the onset of UV illumination was monitored over an extendedperiod of time (20 seconds) so as to ensure that a complete picture wasobtained of the reaction of the sensing material to the UV illumination.However, it will be observed from FIGS. 18A and 18B that significanttransient effects occur within a very short time period followingswitching on of the ultraviolet illumination, notably within the firsttenths of a second after onset of UV illumination. Accordingly, certainsystems and methods embodying this aspect of the invention are designedto give fast results by basing the detection and/or identification of atarget gas on processing of the first part of the sensing material'stransient response, i.e. the response during the first second afteronset of UV illumination.

In the first embodiment of the invention, described above with referenceto FIGS. 3 to 11, the sensing layers of each storey have respectivemeasurement electrodes, so each storey can produce an electrical signalthat contributes to gas discrimination. A specific advantage of gassensors according to the first embodiment of the invention is that thesensors in the multi-storey architecture can be: controlled, dynamicallydriven and acquire data individually.

A second embodiment of the invention will now be described withreference to FIGS. 12 to 15. According to the second embodiment of theinvention a multi-storey chemoresistor type gas sensor is constructed byproviding, in opposed relation to a chemoresistor type gas-sensingelement, a layer of material that can be activated, when desired, toalter the composition of the gas reaching the gas-sensing element.

FIG. 12 illustrates an example of a gas sensor architecture 70 accordingto the second embodiment of the invention. In the gas sensor 70 of FIG.12 a gas-sensing element 71 has the same general structure as the gassensing elements CH1, CH2 of FIGS. 3 to 5. However, in the secondembodiment of the invention this gas-sensing element 71 is no longerarranged in series with a second gas-sensing element, instead it faces asecond storey 72 which bears a layer of material 75 that can beselectively activated so as to change the gas sample before it reachesthe sensing layer of the first storey 71. The material used for thelayer 75 may be a semiconducting metal oxide of the same kind as thematerials used for the sensing layers 52; it is also possible to usecatalytic materials, for example AuTiO₂ or Pt.

As indicated by the arrow marked in dashed lines in FIG. 12, gas flowsthrough the selectively-activatable material 75 of the second storey 72before reaching the sensing layer of the first storey 71. The layer(s)supporting the selectively-activatable material 75 of the second storey72 is (are) porous or provided with suitable holes (e.g. holes 58 as inthe first embodiment) to allow passage of the gas. Accordingly, if theselectively-activatable material is activated, for example using aheater 74 or an ultraviolet light from a source 76 (or any othertechnique appropriate to the nature of the material forming the layer75), then the gas sample will be changed before it reaches the firststorey 71.

It could be considered that the second storey 72 functions rather like acatalytic filter, but an active catalytic filter that can be switched onand off. Although it is known to use filters in association withsemi-conducting metal-oxide gas sensors, the known filters tend to be ofone of two kinds: either filters which operate rather like a sieve,physically blocking the passage of larger gas molecules, or filterswhich consist of a layer of material deposited directly on the sensinglayer and activated whenever the sensing layer is activated (e.g. by aheater). In the second embodiment of the invention theselectively-activatable material 75 of the second storey can beactivated independently of the activation of the sensing layer of thefirst storey 71.

One advantage provided by the second embodiment of the invention is thatit provides an additional technique for increasing the number ofmeasurements that can be taken by the gas-sensing element 71 of thefirst storey. More particularly, the response of the sensing layer ofthe first storey 71 can be measured at times when theselectively-activatable layer 75 of the second storey is activated(possibly at multiple such times with other operating conditions beingvaried between each measurement) and at comparable times when this layer75 is not activated. Accordingly, the analysis of the gas sample can bebased on an increased range of parameters, which tends to improve theaccuracy of the results.

Unlike devices according to the first embodiment of the invention, indevices according to the second embodiment of the invention noelectrical signal is output by the storey that bears the layer 75 ofselectively-activatable material. However, the materials specified foruse in the layer 75 can show very good functionality for chemicalfiltering. Moreover, in this case there is no need to fabricate ameasurement electrode for the storey carrying layer 75 and somanufacturing costs are reduced.

In the second embodiment of the invention a multi-storey structure has asecond storey 72 bearing a selectively-activatable layer 75 that isporous and gas undergoing analysis passes through the porousselectively-activatable layer 75 on the second storey 72 to reach asensing element provided on the subsequent storey 71 of the device.Accordingly, the composition of the gas sample reaching that sensingelement is influenced to a great extent by the material of theselectively-activatable layer 75 present on the preceding storey.

In the first and second embodiments of the invention as described sofar, the gas flow path from one storey in the multi-storey architectureto the next is defined by holes/pores through the membrane 53 carryingthe sensing material 52/selectively-activatable material 75, and themembrane 53 extends across the sensor so that there is no other gas flowpath to the subsequent storey. This arrangement ensures that the gaspassing to the subsequent storey is maximally affected by the sensingmaterial 52/selectively-activatable material 75 of the preceding storey.

The present inventors have realized that a similar effect can beachieved, albeit to a lesser degree, even in variants of the first andsecond embodiments in which the gas undergoing analysis does notnecessarily pass through the selectively-activatable layer but simplycomes into contact with that layer, for example because there is lessthan a total seal between the successive storeys of the gas sensor andsome gas reaches the subsequent storey without passing through theselectively-activatable layer of the preceding storey. Moreover, theinventors have devised variants of the second embodiment of theinvention in which—even though the selectively-activatable material islocated in the same chamber as the sensing material of the subsequentstorey—the gas can be caused to be modified by theselectively-activatable layer before it comes into contact with thesensing layer of the subsequent storey. Accordingly, in these variantsit may be considered that the gas passes in series from one storey tothe next as in the earlier-described embodiments.

A first variant of the second embodiment is illustrated in FIG. 13. Ascan be seen from FIG. 13, according to the variant of the secondembodiment a gas sensor 80 includes a housing 86 containing first andsecond storeys 81, 82 mounted to define a chamber 90 between them. Thefirst storey 81 is a gas-sensing element comparable to one of thegas-sensing elements CH1, CH2 of FIGS. 3 and 4. The second storey 82bears a layer 85 of selectively-activatable material comparable to thatof FIG. 12 but, in the present case, the layer 85 is positioned insidethe chamber 90 facing the sensing layer of the first storey 81.

In this first variant of the second embodiment, the second storey 82 maybe porous, or provided with holes, so that the gas passes through theselectively-activatable material 85 as shown by the arrow A′ marked inFIG. 13 using dashed lines. In such a case it is clear that the gasreaching the first storey 81 can be modified as its passes through theselectively-activatable material 85, notably by activating that material85 (e.g. using an associated heater).

Alternatively (or additionally), the gas sample to be analysed may enterthe chamber 90 through one or more openings 87 in the side wall of thehousing, and exit the chamber 90 through the first storey 81, asindicated by the arrows A shown in FIG. 13. In this case, gas enteringthe chamber 90 through the opening 87 may still be modified to a usefulextent by the selectively-activatable material 85 of the second storey82 before it is detected by the sensing material of the first storey 81by virtue of positioning the material 85 sufficiently close to the firststorey 81 and/or suitable control of the flow rate of gas into/out ofchamber 90 (as explained below).

A second variant of the second embodiment is illustrated in FIG. 14. Inthis variant the gas is introduced inside the chamber 100 under thecontrol of a valve 104 ₁ via a lateral opening 102 and can passpartially (B′) though the first storey 101. As in the first variant ofthe second embodiment, the gas may be modified by the activation ofcatalytic materials 105, before it is detected by the sensing materialin the first storey 101, by suitable control of the spacing between thecatalytic material 105 and the first storey 101 and/or the gas flowrate. The gas exhaust is through an exit opening 103 under the controlof a second valve 104 ₂.

This system can allow the flow rate B′ through the sensor to be adjustedby control of the valves 104. Different patterns of gas flow aresupported including gas flow controlled according to the following twopatterns:

Pattern 1:

-   -   a) introduce gas into chamber 100 by opening valve 104 ₁ while        keeping valve 104 ₂ closed,    -   b) close valve 104 ₁ and maintain gas in chamber 100 during a        selected time period, (during this period some gas species will        exit the chamber 100 via the sensor 101, as indicated by arrow        B′),    -   c) open valve 104 ₂ to empty chamber 100, and    -   d) repeat steps a)-c) as desired.

If desired, the emptying of the chamber 100 in step c) can beaccompanied by (and/or followed by) the passage of a cleaning gas, e.g.air, through the chamber, before a new sample is introduced.

Typically, all the desired measurements in relation to a given gassample are taken while it is resident in the chamber during a single oneof the residence periods mentioned in step b) above; this reduces therisk that operating conditions may vary, independently of the operator'scontrol, between different measurements taken in relation to a given gassample. Depending on the gases under analysis and the sensing materialsin question, this will typically require a residence time of between 1second and several minutes in the chamber 100.

However, it is not essential to perform all measurements during a singleresidence period; if desired, successive portions of a gas sample may beintroduced into the chamber during different instances of step b),notably during successive instances of step b).

Pattern 2:

Allow gas to flow continuously through the device during the measurementperiod. During continuous gas flow, control the opening amount of valves104 ₁ and 104 ₂ to obtain a desired volume of gas flowing through thedevice each minute. The gas flows, primarily, from 102 to 103 when valve104 ₂ is open, but a portion flows through sensor 101 (arrow B′ in FIG.14). Advantageously, the flow rate through valve 104 ₂ is setsufficiently low to allow homogenization of the gas in chamber 100.

Typically, when the thickness of the wafer (dimension t in FIG. 14) isabout 0.5 mm, useful modification of the gas by the catalytic material105 is obtained when the distance between the storeys (dimension s inFIG. 14) is in the range of about 0.5 mm to about 3 mm—corresponding toa volume of 2×s mm³ (i.e. about 1 mm³ to about 6 mm³)—for gas flowaccording to Pattern 1 when the gas is held in chamber 100 for a periodin the range of about 1 second to 30 seconds, and for gas flow accordingto Pattern 2 when the gas flow rate from 102 to 103 during continuousgas flow is in the range of about 1 ml per minute to about 20 ml perminute.

FIG. 15 shows a third variant; this is a typical example of an assemblyin which global flow B is not limited and CH1 CH2 cells with partialFlows B′ and B″ allow measurement of the gas. As in the first and secondvariants of the second embodiment, the gas may be modified by theactivation of catalytic materials 105, before it is detected by thesensing material in the first storey 101, by suitable control of thespacing between the catalytic material 105 and the first storey 101and/or of the gas flow rate.

Arranging the selectively-activatable material of the top storey on theopposite side of a common chamber from the bottom sensor, as shown inFIGS. 13 to 15, can be useful for promoting catalytic combustion oftarget gas because the contact surface area of gas enlarges. As aresult, the combustion time can be monitored to discriminate gases. Thistechnique can be applied also in the first embodiment of the inventionto produce equivalent advantages.

It will readily be understood that the first and second embodiments ofthe invention can be combined, notably to produce a stacked structure inwhich some storeys have sensing layers equipped with measurementelectrodes and one or more other storeys bear sensing layers withoutmeasurement electrodes.

FIG. 19 illustrates some other configurations of gas sensors accordingto the first aspect of the present invention, in which two gas-sensingelements are mounted in series. It will be understood that theconfigurations illustrated in FIG. 19 may be extended by adding otherstoreys and that, once again, such extended structures may includeadditional storeys have sensing layers equipped with measurementelectrodes and/or storeys bear sensing layers without measurementelectrodes, as desired. The curved arrows in FIGS. 19C and D indicatecircular flow, and preferably vortex flow, designed to ensure that eachstorey in the device affects the gas sensed by the other storey (bearingin mind that the activation of the different storeys will be controlledso that they are not necessarily activated at the same time).

Although the present invention has been described above with referenceto particular embodiments, the skilled person will readily understandthat the present invention is not limited by the details of theabove-described embodiments. More particularly, the skilled person willunderstand that various modifications and developments can be made inthe above-described embodiments and that different embodiments can bedesigned without departing from the present invention as defined in theappended claims.

For example, although the specific embodiments described above make useof a base substrate made of silicon it will readily be understood thatother materials having adequate mechanical properties could be usedinstead: including but not limited to ceramic materials (Al₂O₃, etc.),glass, and plastics.

Furthermore, although the description above refers to the use of layersof catalytic materials, it is to be understood that the layers with orwithout measurement electrodes may be formed of “trap” materials, forexample Tenax®, zeolites, activated charcoal, etc., and these materialsmay be passive (rapid trapping, slow release) or active (e.g. wheredesorption is produced using heating). It is to be understood that, indevices according to the first aspect of the invention, passivematerials will be employed in architectures in which at least one storeyincludes a selectively-activatable layer.

Moreover, although the use of heating pulses to promote baseline returnis described above in the context of a particular embodiment and aspectof the invention, it is to be understood that heating pulses may beemployed for this purpose in the other embodiments and aspects of theinvention.

Similarly, although the use of a cleansing flow of air (or othercleaning gas) through a chamber in a gas sensor is described above inthe context of a particular aspect of the invention, it is to beunderstood that a cleaning phase of this type may be employed in theother embodiments and aspects of the invention. Furthermore, usefulmeasurements may be taken during such a cleaning phase (as gas speciesdesorb from the catalytic and/or sensing material layer(s)) and/orduring phases where the gas sample is diluted.

What is claimed is:
 1. A gas sensor of chemoresistor type having amulti-storey structure, the sensor comprising: a first storey comprisinga sensing layer provided on an insulating layer, and measurementelectrodes provided in contact with the sensing layer; a second storeycomprising a support layer supporting a second sensing layer that is alayer of selectively-activatable material spaced from the sensing layerof the first storey, and second measurement electrodes in contact withsaid second sensing layer; wherein the first storey and the secondstorey each constitute a chemoresistor type gas sensing element, and anactivation unit for controlling activation of theselectively-activatable layer; wherein the first and second storeys arearranged in series so that gas passes successively through the secondstorey to the first storey and the gas reaching the sensing layer of thefirst storey can be modified by activating the selectively-activatablematerial of the second storey.
 2. A gas sensor according to claim 1, andcomprising: control means adapted to take measurements via themeasurement electrodes of the first storey at times when the layer ofselectively-activatable material of the second storey is activated andde-activated, respectively, and to process said measurements todiscriminate a gas presented to the gas sensor.
 3. A gas sensoraccording to claim 2, wherein: holes or pores are formed through eachstorey for permitting the passage of gas from one storey to the next. 4.A gas sensor according to claim 3, comprising a stack of chemoresistortype gas sensing elements, wherein different materials form therespective sensing layers of at least two different storeys in thestack.
 5. A gas sensor according to claim 3, wherein each gas sensingelement has a micro-hotplate structure.
 6. A gas sensor according toclaim 2, wherein: the first and second storeys define a chambertherebetween, and the sensing layer of the first storey and theselectively-activatable layer of the second storey are disposed onopposed surfaces of the chamber.
 7. A gas sensor according to claim 1,wherein: holes or pores are provided through the second storey at leastin a region where the selectively-activatable layer is provided, forpermitting the passage of gas through the selectively-activatable layerof the second storey to the sensing layer of the first storey.
 8. A gassensor according to claim 2, wherein the activation unit comprises aheater.
 9. A gas sensor according to claim 2, wherein the activationunit comprises a source of ultra-violet light operable to expose theselectively-activatable layer to pulses of ultraviolet light, and thegas sensor comprises a setting unit configured to control a duty ratioor duration of the ultraviolet light pulses applied by the source ofultraviolet light, in dependence on a target gas species to be detectedby the gas sensor or an interfering gas species.
 10. A gas sensoraccording to claim 2, wherein the activation unit comprises a source ofultraviolet light operable to expose a sensing layer in the gas sensorto pulses of ultraviolet light, and the gas sensor comprises a settingunit configured to control a duty ratio or duration of the ultravioletlight pulses applied by the source of ultraviolet light, in dependenceon a target gas species to be detected by the gas sensor.
 11. A gassensor according to claim 1, comprising: a source of ultraviolet lightoperable to expose one or more sensing layers in the sensor toultraviolet light, and an analysis unit configured to analysemeasurements taken via measurement electrodes in contact with said oneor more sensing layers to determine the transient response of said oneor more sensing layers at onset of application of ultraviolet light.